An arboricultural impact assessment (AIA) is a report that takes your proposed development layout and assesses its effect on the surveyed trees — which are removed, which are kept, and where construction conflicts with their root protection areas — and it is the document your local planning authority most often demands with the application itself.
If a tree survey answers "what have we got?", the AIA answers "what does building this do to them?" It is the pivot point of the whole BS5837 process: the moment the design meets the trees. Get it right and validation is a formality; get it wrong — or skip it — and your application can be returned before a case officer even looks at it.
When does the LPA demand an AIA?
There is no single national law that says "an AIA is required." The compulsion comes from three separate places, and it is worth keeping them distinct because people constantly conflate them:
- Local validation requirements — most councils list arboricultural information on their validation checklist where a proposal is on or near trees, or "likely to affect trees." Miss it and the application is returned as invalid before it is even assessed.
- Planning conditions — permission is frequently granted subject to further tree detail such as an arboricultural method statement, but the AIA itself typically comes before that, at submission.
- Statutory protection — Tree Preservation Orders and conservation-area rules are genuine law, criminal-law backed, and sit on top of everything else.
In practice, the trigger is proximity. If any tree — on your land or overhanging from next door — is close enough that its root protection area touches the works, expect the council to want an AIA. Crucially this includes driveways, patios, drainage runs and utility trenches, not just the building footprint. Guildford Borough Council's validation wording is typical: arboricultural information is required where "there are trees on or overhanging the site" or "the work will impact trees" — and it explicitly names neighbouring trees whose RPAs extend into the site.
Where the AIA sits in the BS5837 stack
The AIA is not a standalone document; it is the third stage of a sequence. Understanding the order is the single most useful thing you can learn here, because getting it wrong is expensive.
| Stage | Document | When |
|---|---|---|
| 1 | Tree Survey + Tree Constraints Plan | Ideally before the layout is designed |
| 2 | Arboricultural Impact Assessment (AIA) | Once a layout exists — submitted with the application |
| 3 | Tree Protection Plan (TPP) | With the application, and/or to discharge a condition |
| 4 | Arboricultural Method Statement (AMS) | Usually a pre-commencement condition |
The AIA, TPP and often a preliminary AMS travel together as the pre-permission package — they are the "can we build this?" documents. The detailed AMS and final TPP are the post-permission "prove you'll protect them" documents, discharged as conditions after you already hold permission. See our BS5837 explainer and the wider arboricultural report family for the full picture.
What's inside an AIA
A competent AIA, written to BS5837:2012, works methodically through the collision between design and trees:
- A schedule of trees to be removed to enable the development, cross-referenced to the survey and to each tree's BS5837 category (A, B, C or U). Losing a Category C tree is a footnote; losing a Category A tree is a planning-balance issue.
- A schedule of retained trees, and an assessment of the pressures they will face — direct construction damage, and longer-term "post-development pressure" such as shading of future gardens or windows that leads occupiers to demand felling a year after they move in.
- Root protection area incursions — exactly where the proposed footprint, hard surfacing, level changes or services cut into an RPA, expressed as an area and a percentage, and whether that incursion can be engineered around (no-dig surfacing, special foundations) or whether the design must change.
- The arboricultural implications of the layout — access routes, level changes, cut-and-fill, excavation, and the effect on any protected trees.
- Mitigation and recommendations, including the outline protection measures that the TPP and AMS then develop into enforceable detail, plus replacement planting where trees are lost.
The AIA should be honest about losses. A tree officer who spots an over-optimistic assessment — a Category A tree quietly downgraded to B or C so it can be removed with less fuss — will reject it, and you lose weeks re-submitting.
How the root protection area drives everything
The RPA is the technical heart of the assessment. It is the notional below-ground rooting area that must be safeguarded from excavation, compaction, level change and contamination. Under BS5837:2012 it is calculated as a circle with radius = 12 × the stem diameter measured at 1.5 m above ground, with the area capped at 707 m² (a 15 m radius) for very large trees. Where rooting is provably asymmetric — constrained by an existing building or road on one side — the RPA can be re-plotted as a polygon of the same area rather than a circle.
| Stem diameter | RPA radius (12×) | Approx. RPA area |
|---|---|---|
| 200 mm | 2.4 m | ~18 m² |
| 350 mm | 4.2 m | ~55 m² |
| 500 mm | 6.0 m | ~113 m² |
| 620 mm | 7.4 m | ~172 m² |
| 900 mm | 10.8 m | ~366 m² |
| 1,250 mm+ | capped at 15.0 m | 707 m² (cap) |
Veteran and ancient trees are a category apart: their RPA is 15 × stem diameter, or the canopy edge plus 5 m, whichever is larger, and they are treated as irreplaceable habitat under national planning policy. An incursion there is not a technicality to be engineered around — it is a fundamental objection.
Worked example: a rear extension near a neighbour's oak
Say your site has a mature oak — actually rooted in next door's garden — with a stem diameter of 500 mm measured at 1.5 m. Its RPA is a circle with radius 12 × 0.5 = 6 m (about 113 m²), and roughly a third of that circle projects across the boundary onto your land.
Your proposed single-storey rear extension clips the edge of that 6 m circle. The AIA's job is to quantify the incursion and judge it. If the footprint takes, say, 8 m² out of a 113 m² RPA — around 7% — that sits comfortably under the roughly 20% of RPA that BS5837 treats as the threshold for concern, and can usually be justified with a no-dig foundation (screw piles or a beam-and-pad design) that avoids severing roots. That method then gets specified in the AMS and drawn on the TPP.
If instead the extension drove through 30–40% of the RPA, straight through the heart of the rooting zone, the honest AIA conclusion is that the design has to move. Discovering that at AIA stage costs a redesign; discovering it after a refusal costs the redesign plus the wasted determination period. Because the tree is the neighbour's, you would also have no ability to fell it as an alternative — its retention is effectively fixed.
What goes wrong: the most expensive mistake
BS5837 is design-led. The intent is that the tree survey and constraints plan shape the layout, not the other way round. The classic and most costly failure is to fix the design first, then commission the AIA as a box-ticking afterthought, only to find roads, foundations or drainage running straight through an RPA. At that point your choices are an expensive redesign or a likely refusal.
Other recurring failure modes worth avoiding:
- Ignoring off-site trees whose RPAs cross the boundary — a survey and AIA that stop at your fence line will be sent back.
- Unjustified RPA incursions — proposing hard surfacing or foundations inside an RPA with no engineered solution offered.
- Optimistic categorisation the tree officer rejects on their own site visit.
- Not flagging TPO or conservation-area status, treating a legally protected tree as freely removable.
Commission the survey and constraints plan early, let them inform the layout, and the AIA becomes a confirmation rather than a crisis.
Requirements are set locally — check your own council
This is the part generic guides skip. There is a national baseline for what makes a planning application valid, but the arboricultural specifics are a local overlay set by your individual local planning authority. What varies council to council: the exact trigger wording and any distance threshold; whether they demand a full AIA, TPP and preliminary AMS upfront or accept a survey at validation and condition the rest; local TPO density and conservation-area coverage; the presence of ancient woodland or veteran trees that raise the bar; and how strictly the local tree officer reads an assessment in practice.
So before you commission anything, read your authority's validation checklist. You can see how the requirement is worded in different places — for example Leeds, Manchester and Bristol each publish their own arboricultural triggers, and a leafy conservation-heavy borough like Lambeth tends to scrutinise tree impact far harder than a low-tree-density urban authority. PlanWatch tracks live tree-related planning activity per council, so you can check what applications near you are being submitted, conditioned and refused before you spend a penny. Browse your area from the tree-surveys hub.
What it costs and how long it takes
As a rough guide, a standalone AIA added to an existing survey often starts from around £200, while a combined survey + AIA for a typical development is commonly £500–£1,000. Larger, tree-heavy or phased sites run to £1,500 and beyond. Turnaround for a straightforward site is often one to two weeks from instruction — a site visit plus desk write-up. BS5837 assessment does not need leaf-on conditions, so surveys can be done year-round, though if bat or nesting-bird ecology surveys are triggered alongside, those have tight seasonal windows that can dominate the timeline. See our full cost breakdown for the wider package, and how to find a qualified surveyor so the report carries weight with the tree officer.
Scotland, Wales and Northern Ireland
BS5837:2012 applies UK-wide as a British Standard, so the AIA itself looks the same wherever you build. What differs is the statutory backdrop: Scotland works under the Town and Country Planning (Scotland) Act 1997, and Wales and Northern Ireland each have their own parallel regimes and validation arrangements. The validation-list mechanism and the practical need for an AIA are broadly the same, but the trigger wording, timescales and tree-protection detail differ — always check the specific local authority's requirements before you submit.